Tests on virgin W coatings

Experiments were carried out to characterize the ablation properties of samples prepared by different methods. LIBS profiles were recorded at a fixed value of fluence. DIARC (Table 2.1, samples 2, 3, 4) and IAP samples (Table 2.2 samp-les 1, 2, 4) were tested. Results are presented in [58] and [59].

The spectra were recorded with two different spectrometers from different directions: Mechelle 5000 spectrometer looking at 45º to the target normal (Figure 2.2, position 2); MDR-23 looking with the entrance slit at 90º to the target normal (Figure 2.2, position 3).

SEM images of the samples are in the Figure 3.5. Pictures show that com-pared with the fine structure of DIARC sample, the “smooth” IAP sample has remarkably larger structural elements. Even larger structures are present on the rough IAP samples.

Figure 3.5. SEM images of the tested samples. a) DIARC b) IAP, smooth c) IAP, rough.

Figure 3.6. XRD spectra of the tested samples and bulk Mo. Intensity is in log scale and the baselines are shifted for clarity(originally the background levels are approximately equal).

Recorded XRD spectra (Figure 3.6) show that DIARC samples have narrower lines than smooth IAP samples. That refers to larger crystallites in DIARC coatings. Nevertheless, results may be affected by Mo lines from the substrate.

Mo lines are situated near the W lines and may cause apparent line broadening.

Rough IAP samples have additional diffraction lines in the range 60–70º, these lines are not present in other spectra. According to the literature [60] these lines

belong to the W metastable β-phase (other lines belong to more common α-phase).

Domestic LIBS setup described in 2.1 was used to carry out the LIBS spectra recording and depth profiling. The laser lased at 532 nm. The laser energy at the sample surface was 80–90 mJ and spot diameter was 0.7 mm, the corresponding average fluence Φ was 7 J/cm².

Figure 3.7. Comparison of the measured spectra of the DIARC and IAP samples.

The LIBS spectra in 385–415 nm range were recorded by MDR-23 spectro-meter. Examples of the measured spectra are in the Figure 3.7. To build ele-mental depth profiles strong spectral lines at 390.2 nm (Mo I) and 400.9 nm (W I) were used. Comparison of the elemental depth profiles for the DIARC samples is in the Figure 3.8. To reduce the effect of shot-to-shot fluctuations, trendlines of “moving average” were used. The depth profile for W is not steep and within the measurement uncertainty these profiles for different samples are comparable. This is likely to the crater effects related to the laser beam shape [42], flaking of the coatings etc. The first laser shots recorded at specific site lack of Mo signal, indicating that the laser crater is fully in the W coating. That plateau was used to calculate the ablation rate for the coatings. For pure W coating the plateau is around 29 laser shots, corresponding to ablation rate 69 nm/shot. For the samples containing 10 and 40% Al the ablation rates are 74 nm/shot and 285 nm/shot, respectively. We can see that high Al content increases drastically the ablation rate for the coatings.

Figure 3.8. Elemental depth profiles (W and Mo) for the DIARC samples. Lines are smoothed using the “moving average”. The experimental points in the graph are presented for the sample with 40% Al .

Results of experiments on the IAP samples are in the Figure 3.9. It should be noticed that the x-axis has different scale than in previous graphs for DIARC samples. For pure W coatings a Mo signal well above the noise level is detect-able already from the second laser shot. Controversially, for the Al containing samples Mo signal came clearly detectable during the 3–4th laser shot. The continuous background radiation was remarkably stronger during ablating the coating, compared to the continuous background signal obtained from the Mo substrate. That likely indicates enhanced laser radiation absorption and thus more porous structure of the coatings. This is also supported by the SEM images. Due to the small number of laser shots needed to go through the coating, the uncertainty for the ablation rate is high; the ablation rate is roughly in the range of 500 nm/laser shot.

In addition to the intensity of W and Mo spectral lines the total intensity in the measured spectral range was calculated. Figure 3.10 shows that for the DIARC samples (except the one with 40% Al content) this value is nearly inde-pendent of the laser shot number. For IAP samples the radiation of the laser-induced plasma generated from the coating is much more intense compared to the one generated from the substrate. This effect could be explained by the enhanced absorption of the laser radiation and smaller thermal conductivity of the IAP coatings.

Figure 3.9. Elemental depth profiles of W (left) and Mo (right) for the IAP samples.

Figure 3.10. Total intensity in the selected spectral region.

Experiments on W coatings with different surface morphology and structure (caused by the production methods) revealed that these parameters have very strong effect on the ablation rate and also on the measured LIBS spectra.

To apply LIBS for in-situ erosion and deposition measurements methodo-logy for taking into account the change in the ablation rate should be developed.

One possible approach that should be investigated is to use the total intensity or background intensity as a normalization parameter.

3.1.4. D detection from W and W/Al mixture coated samples